1. Field of the Invention
This invention generally relates to printed document reproduction and, more particularly, to a system and method for printing documents with an improved ability to recover from document assembly failures.
2. Description of the Related Art
FIG. 1 is a schematic block diagram depicting a conventional printer with dual paper and print paths (prior art). The demands for higher speed printing are approaching a practical maximum in the throughput of the existing print engines, firmware, and paper path mechanisms. One solution for higher print throughput is to put two or more print engines together in a single cabinet such that they can be operated concurrently and independently, as shown.
The print engines can be identical, or of different speeds and capabilities. For example, to minimize the cost of having two color print engines in a single color printing system, one print engine can be limited to printing the sheets that are exclusively monochrome. Although the interim stacking bins are depicted as first in-first out (FIFO), in an alternate configuration (not shown) the bins may be last in-first out (LIFO).
Each printing engine has its own paper path and may run at different speeds. The problem of collating the final output can be resolved by keeping track of the page order of the sheets, stacking the printed pages in individual bins, and then merging the printed sheets just prior to putting the sheets into the output bin. There is no need to wait until the current job is finished in order to start a new job. Any print engine that has completed its printing can start printing sheets from the next job, even before the other engines have completed printing sheets from the current job.
FIG. 2 is a schematic block diagram depicting a specific failure mode associated with the hybrid printing systems of FIG. 1 (prior art). The problem concerns the recovery of pages when a failure occurs in the final assembly and output stacking of the document. Consider, for example, that each print engine stacks their printed pages in separate, interim bins awaiting the collation (merging) of the individual sheets into a single document for placing in the output bin. If a paper jam were to occur just prior to the final output stage (see FIG. 1), then one or all of the interim bins would have many already-printed pages. It is possible that one or all of the bins may have hundreds of printed sheets that cannot be outputted because the damaged sheets are missing. On the other hand, if just the damaged sheets are reprinted, they must somehow be advanced to the output bin before any additional drawing sheets are supplied from the interim bin(s).
Ideally, just the damaged pages are reprinted, and the print job can be continued without restarting the print job from page one. The problem, therefore, is to print and insert clean copies of the damaged sheets in the correct place within the final document, while saving the existing clean sheets already printed stored and in the temporary bins.
Conventionally, the problem is solved by requiring the user to throw out and reprint the pages not already in the output bin. To address this problem, some printer manufacturers build devices that print pages sequentially. First, all the pages from the color engine are printed, then the color-engine pages are inserted into the final bin in the required order as the monochrome engine is printing, making it feasible to halt the monochrome engine and have it re-print some pages if they get stuck in the path to the final bin.
Some of the above-mentioned printing problems also occur in printers employing a single print engine. Conventionally, when a paper jam or other failure occurs, all printing halts until the situation is remedied by human intervention. After the problem has been fixed, only then does the printing continue. Some of these problems could be solved without human intervention if a method of recovery was provided. Secondly, having the job halt until the problem is fixed creates an enormous inconvenience in having to restart the job at the point of failure. For example, if page 3 of a 500 page printout jams, the user will have to correct the problem, and then wait for 497 pages to print. If the printer could continue printing, or continue performing the bulk of the work before the paper jam was fixed, this would improve greatly the convenience of the printer.
Thus, for printers with multiple print engines as well as single print engine printers, it would be advantageous if a means existed for recovering a failed printer/collation job by reprinting just the damaged sheets.